1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the present invention relates to a nonvolatile semiconductor memory device including a variable resistive element formed by sequentially stacking a lower electrode, a variable resistor with a perovskite-type crystal structure, and an upper electrode.
2. Description of the Related Art
As next generation nonvolatile random access memories (NVRAM) capable of operating at high speed, that replace flash memories, there have recently been proposed memories with a variety of device structures, such as FeRAM (Ferroelectric RAM), MRAM (Magnetic RAM) and OUM (Ovonic Unified Memory). The developments of those memories have been intensely competitive in terms of enhanced performance, greater reliability, lower cost and more excellent process matching. However, each of those memories has its merits and demerits. Hence there is a long way to go before a “universal memory”, an ideal memory having all the merits of the above-mentioned memories, can be realized.
For example, the FeRAM, which has been already in practical use, is produced utilizing a phenomenon of spontaneous polarization inversion of a ferroelectric oxide. The FeRAM is superior in low electric power consumption and high-speed operation, but inferior in high cost and destructive reading. The MRAM uses a ferromagnetic tunnel effect element utilizing a giant magnetoresistance (GMR). The ferromagnetic tunnel effect element has a structure in that two ferromagnetic material layers made of Fe, Co, Ni or the like are sandwiched between extremely thin insulating layers (tunnel barrier layers) made of Al2O3 or the like, and controls magnitude of a tunnel current that flows through the insulating layer by changing the direction of magnetization (spinning) of the ferromagnetic material layers, to express effects of the memory. However, the ferromagnetic tunnel effect element has a major problem of high electric power consumption in inversion of magnetization at the time of programming, as well as micronization. Further, the OUM, which is produced on the basis of thermal phase transformation of a chalcogenide material, is superior in low cost and process matching, but inferior in micronization and high-speed operation since it thermally operates.
As opposed to the above conventional art, there is a method of applying a voltage pulse to a perovskite material, which is known to have a colossal magnetoresistance effect, to reversibly change electric resistance. This method is disclosed in U.S. Pat. No. 6,204,139 and “Electric-pulse-induced reversible resistance change effect in magnetoresistive films”, Liu, S. Q. et al., Applied Physics Letter, Vol. 76, pp. 2749–2751, 2000, written by Shangquing Liu, Alex Ignatiev and others, University of Houston, USA. This method is considerably revolutionary in that a resistance change by several digit figures is expressed at room temperature without application of a magnetic field while using the perovskite material known to have the colossal magnetoresistance effect. An RRAM (Resistance Random Access Memory) uses a variable resistive element utilizing this phenomenon. Unlike the MRAM, the RRAM requires no magnetic field and, thus, has extremely low electric power consumption, facilitating micronization as well as high integration, and the dynamic range of the resistance change of the RRAM is markedly wider than that of the MRAM, thereby to obtain multi-level storability. A basic structure of an actual device is extremely simple. As illustrated in FIG. 12, the device is configured by sequentially stacking, in a direction vertical to a substrate, a lower electrode material 21, a perovskite material 22 and an upper electrode material 23. In the element structure illustrated in FIG. 1, the lower electrode material 21 is formed of a film of yttrium-barium-copper oxide, YBa2Cu3O7 (YBCO), deposited on a single crystal substrate 24 of lanthanum-aluminum oxide, LaAlO3 (LAO), the perovskite material 22 is formed of a film of crystalline praseodymium-calcium-manganese oxide, Pr1—xCaxMnO3 (PCMO), and the upper electrode material 23 is formed of a film of Ag deposited by sputtering. This storage element operates to change the resistance of the perovskite thin film which is sandwiched between the upper and lower electrodes 21 and 23 by controlling a polarity, a voltage and a pulse width of an electric pulse to be applied to between the two electrodes. A resistance value changed by the pulse application is stored for a long period of time after the application, and a nonvolatile memory function can be obtained by, for example, setting the low resistance state to “0” and the high resistance state to “1”.
However, in trying to change the resistance of the perovskite thin film in the element structure as illustrated in FIG. 12, the operation voltage is as high as over ten volts to dozens of volts if referring to U.S. Pat. No. 6,204,139. Such a high operation voltage is practically problematic. Lowering the operation voltage has been considered possible by reducing the thickness of the perovskite material film that constitutes the variable resistive element. However, the present inventors have clarified the fact that reduction in thickness of the perovskite thin film in the same element area and the same electrode area causes a very low impedance, a resistance value of 10Ω or less, of the variable resistive element, leading to insufficient capacity to supply current in a commonly usable pulse power source or pulse application circuit. Thereby application of sufficient voltage to the variable resistive element becomes difficult.